1. Field of the Invention
The present invention relates to a wavelength-variable light source used in the field of optical communication, as well as to a device for measuring optical component loss using the wavelength-variable light source.
2. Description of the Related Art
FIG. 14 shows a wavelength-variable light source in the related-art. An antireflection coating 1A is provided on one side of a semiconductor laser 1, and the light exiting from the end covered with the antireflection coating 1A is collimated by a lens 5. The thus-collimated light is subjected to wavelength selection by a diffraction grating 2, and the thus-selected light returns to the diffraction grating 2 by means of the mirror 3. The light is again subjected to wavelength selection by the diffraction grating 2, and the light is fed back to the semiconductor laser 1, thereby effecting lasing.
The light output from the other end of the semiconductor laser 1 is converted into collimated light by means of a lens 6. After having passed through an optical isolator 8, the thus-collimated light is converged on an optical fiber 4 by means of a lens 7.
The light source shown in FIG. 14 is called a Littman type. During one round trip, the light is subjected to two rounds of wavelength selection by the diffraction grating 2. The light source is superior in wavelength selection characteristic and has been known as one of the most popular methods.
In such a related-art wavelength-variable light source, as indicated by xe2x80x9coutput from ordinary wavelength-variable light sourcexe2x80x9d shown in FIG. 11, the light that is output when the wavelength of the light is changed assumes an arch-shaped profile, because of gain distribution of the semiconductor laser 1. Here, the characteristic of an xe2x80x9coutput from an ordinary wavelength-variable light sourcexe2x80x9d is determined by means of consecutively plotting outputs P produced at respective wavelengths xcex.
In a case where a transmission loss of an optical component using such a light source is measured, a loss is measured by means of making an optical output flat (without a necessity of any reference), as shown in an example characteristic provided in an upper left position in FIG. 8.
If a drive current of the semiconductor laser is subjected to auto power control (APC) for making an output flat, a drive current If must be changed greatly, as shown in FIG. 13A.
A change in current induces mode hopping or multi-mode lasing, thus rendering the lasing state of the light source unstable. As a result, there may be a case where correct measurement of transmission loss become impossible.
Even if a light-variable attenuator is provided at the output side of the optical component, high-speed tracking of the output is difficult.
An object of the present invention is to provide a wavelength-changeable light source equipped with a semiconductor laser having one end surface covered with an antireflection coating and enhancing the flatness of an optical output.
Another object of the present invention is to enable stable measurement of transmission loss of an optical element with a smaller change in current even when APC is effected with use of a drive current and with less instability of the light source, through use of a wavelength-variable light source which produces an optical output of high flatness.
To solve the above-described problem, the present invention provides a wavelength-variable light source equipped with a semiconductor laser whose one end face is provided with an antireflection coating, the light source comprising:
a power correction filter which permits transmission of the light output from the semiconductor laser, thereby rendering the characteristics of the output light substantially flat.
According to the present invention, the wavelength-variable light source is provided with a power correction filter which permits transmission of the light output from the semiconductor laser whose one end face is provided with an antireflection coating, thereby improving the flatness of the output light.
Preferably, as shown in FIG. 1, the wavelength-variable light source is an external-resonator-type laser light source which converts the light output from the end face covered with the antireflection coating of the semiconductor laser into collimated light, returns the light to a wavelength selection optical element by means of a mirror after having subjected the light to wavelength selection by the wavelength selection optical element, subjects the light again to wavelength selection by the wavelength selection optical element, feeds back the light to the semiconductor laser, and converges and outputs light output from the other end face of the semiconductor laser on and to an optical fiber, a power correction filter being interposed between the semiconductor laser and the optical fiber.
Preferably, as shown in FIG. 1, a diffraction grating is employed as the wavelength selection optical element.
Preferably, as shown in FIG. 2, a beam splitter 9 is interposed between the semiconductor laser land the diffraction grating 2 for extracting a portion of diffracted light which is fed back from the diffraction grating 2 to the semiconductor laser, and the diffracted light extracted by the beam splitter 9 is converged on and output to the optical fiber 4 after have been caused to pass through the power correction filter 10.
Preferably, as shown in FIG. 2, the wavelength-variable light source further comprises a rotation mechanism (designated by arrows) which changes a selected wavelength by means of a change in the angle of the mirror 3.
Preferably, as shown in FIG. 3, avariable band pass filter 11 is used as the wavelength selection optical element.
Preferably, as shown in FIG. 4, a partial reflection mirror is employed as the mirror, and a portion of the light having been fed back to the semiconductor laser 1 is converged on and output to the optical fiber 4.
Preferably, a beam splitter is interposed between the semiconductor laser and the variable band pass filter 11 for extracting a portion of diffracted light which is fed back from the variable band pass filter to the semiconductor laser, and the diffracted light extracted by the beam splitter 14 is converged on and output to the optical fiber 4 after have been caused to pass through the power correction filter 10.
Preferably, as shown in FIG. 6, the power correction filter is a power correction film 4 provided on the end face of the optical fiber.
Preferably, as shown in FIG. 8, a wavelength-variable light source is provided in a light source section and corresponds to an optical component loss measurement device which connects the light output from the light source to an optical component 21 to be measured, and measures a transmission loss in the light having passed through the optical component 21, wherein the light source section is a wavelength-variable light source 20 having any one of the above-described power correction filters 10.
Preferably, the optical component loss measurement device connects the light output from the wavelength-variable light source having the power correction filter to an optical component to be measured and measures a transmission loss in the light having passed through the optical component. Hence, the light source is subjected to small current variation even during APC of a drive current. Thus, instability of the light source is diminished, thereby enabling stable measurement of a transmission loss in an optical component.
Preferably, a shown in FIG. 8, the optical component loss measurement device further comprises:
a light-receiving section 31 for receiving light having passed through the optical component 21; and
a control section 32 for computing a transmission loss in the light having passed through the optical component from information about the wavelength of the light output from the wavelength-variable light source 20 having the power correction filter 10 and from information about the power of the light output from the light-receiving section 31 and outputting a computation result, wherein the control section 32 controls a drive current to be delivered to the wavelength-variable light source having the power correction filter 10.
Preferably, as shown in FIG. 9, the optical component loss measurement device further comprises an optical spectrum analyzer 33 for receiving the light having passed through the optical component 21, in which the optical spectrum analyzer 33 and the wavelength-variable light source 20 having the power correction filter 10 perform sweeping simultaneously.
Preferably, as shown in FIG. 10, the component loss measurement device further comprises an optical power meter 34 for receiving the light having passed through the optical component 21, and a control section 35 for computing a transmission loss in the light having passed through the optical component, from the information about the power of light output from the optical power meter 34, wherein the control section controls a drive current to be supplied to the wavelength-variable light source having the power correction filter.